1. Field of the Invention
The present invention relates to a direct electrostatic printing method, in which a stream of computer generated signals, defining an image information, are converted to a pattern of electrostatic fields on control electrodes arranged on a printhead structure, to selectively permit or restrict the passage of toner particles through the printhead structure and control the deposition of those toner particles in an image configuration onto an image receiving medium.
2. Description of the Related Art
Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography wherein latent electrostatic images formed on a charged retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
Another form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the above mentioned xerographic form, in that toner is deposited in image configuration directly onto plain paper. The novel feature of DEP printing is to allow simultaneous field imaging and toner transport to produce a visible image on paper directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as it is required in electrophotographic printing.
A DEP printing device has been disclosed in U.S. Pat. No. 3,689,935, issued Sep. 5, 1972 to Pressman, et al. Pressman, et al., disclose a multilayered particle flow modulator comprising a continuous layer of conductive material, a segmented layer of conductive material and a layer of insulating material interposed therebetween. An overall applied field projects toner particles through apertures arranged in the modulator whereby the particle stream density is modulated by an internal field applied within each aperture.
A new concept of direct electrostatic printing was introduced in U.S. Pat. No. 5,036,341, granted to Larson, which is incorporated by reference herein. According to Larson, a uniform electric field is produced between a back electrode and a developer sleeve coated with charged toner particles. A printhead structure, such as a control electrode matrix, is interposed in the electric field and utilized to produce a pattern of electrostatic fields which, due to control in accordance with an image configuration, selectively open or close passages in the printhead structure, thereby permitting or restricting the transport of toner particles from the developer sleeve toward the back electrode. The modulated stream of toner particles allowed to pass through the opened passages impinges upon an image receiving medium, such as paper, interposed between the printhead structure and the back electrode.
According to the above method, a charged toner particle is held on the developer surface by adhesion forces, which are essentially proportional to Q.sup.2 /d.sup.2, where d is the distance between the toner particle and the surface of the developer sleeve, and Q is the particle charge. The electric force required for releasing a toner particle from the sleeve surface is chosen to be sufficiently high to overcome the adhesion forces.
However, due to relatively large variations of the adhesion forces, toner particles exposed to the electric field through an opened passage are neither simultaneously released from the developer surface nor uniformly accelerated toward the back electrode. As a result, the time period from when the first particle is released until all released particles are deposited onto the image receiving medium is relatively long.
When a passage is opened during a development period t.sub.b, a part of the released toner particles do not reach sufficient momentum to pass through the aperture until after the development period t.sub.b has expired. Those delayed particles will continue to flow through the passage even after closure, and their deposition will be delayed. This in turn may degrade print quality by forming extended, indistinct dots.
That drawback is particularly critical when using dot deflection control. Dot deflection control consists in performing several development steps during each print cycle to increase print resolution. For each development step, the symmetry of the electrostatic fields is modified in a specific direction, thereby influencing the transport trajectories of toner particles toward the image receiving medium. That method allows several dots to be printed through each single passage during the same print cycle, each deflection direction corresponding to a new dot location. To enhance the efficiency of dot deflection control, it is particularly essential to decrease the toner jet length (where the toner jet length is the time between the first particle emerging through the aperture and the last particle emerging through the aperture) and to ensure direct transition from a deflection direction to another, without delayed toner deposition.
Therefore, in order to achieve higher speed printing with improved print uniformity, and in order to improve dot deflection control, there is still a need to improve DEP methods to allow shorter toner transport time and reduce delayed toner deposition.
Additionally, in order to ensure entire coverage of the print area, the apertures are preferably aligned in several parallel rows arranged at a slight angle to each other, such that each aperture corresponds to a specific addressable area on the information carrier. The control electrode for each aperture is disposed around the aperture and encompasses an area greater than the aperture. When active, the control electrode has a release area, defined as the area in which toner is drawn from the toner carrier. Because the control electrode is disposed around the aperture, the release area is larger than the aperture diameter.
When printing a solid black surface, the amount of toner available decreases from row to row of apertures. When the release area of the apertures is too large, release areas of consecutive apertures overlap resulting in dots printed "downstream" having a lower density because of an insufficient amount of toner. Having an insufficient amount of toner downstream is known as "toner starvation." Toner starvation causes a degradation of the print uniformity because the dot density becomes dependent on which row the dots are printed through. Toner starvation results in printed surfaces which appear to be striped.